The invention relates to a dry scrubbing method for removing sulfur oxides from hot gases. In one aspect, the invention relates to a dry scrubbing method for removing sulfur oxides and particulate contaminants, such as fly ash, from hot flue gases from combustion of sulfur-containing fossil fuels.
Waste gases discharged from many industrial processes contain sulfur oxides, principally as sulfur dioxide. Flue gases from power plants or the like burning fossil fuels, particularly sulfur-containing coals, can have as much as 3,000 ppm sulfur dioxide, 30 ppm sulfur trioxide and substantial amounts of particulate matter, principally fly ash. Government regulations have made control of air pollution from sulfur oxide and particulate matter increasingly urgent.
Wet scrubbing of flue gases with an alkaline slurry to remove sulfur oxides has been used. While reasonably effective for many applications, the resulting waste product is a sludge which may contain up to 50% water. Handling and disposal of this wet sludge can be quite difficult and costly. It usually requires on-site settling ponds or transportation to remote disposal sites.
To eliminate this and other drawbacks of wet scrubbing, it has been proposed to use a dry scrubbing technique by which an atomized solution or slurry of an alkaline reactant, such as lime, is mixed with the flue gas in a spray dryer. The flue gas acts as the "drying" or hot gas source for the spray dryer. As the water evaporates and the flue gas cools during the drying process, the sulfur dioxide in the flue gas is absorbed on the surface of the minute droplets of the spray and the alkaline reactant reacts with the sulfur dioxide to form sulfide and sulfate compounds. A powder-like mixture of the sulfite/sulfate compounds, residual unreacted alkaline reactant and fly ash is formed. The flue gas entrained with this particulate mixture is exhausted from the spray dryer and passed through dust removal equipment, such as a cloth bag filter or an electrostatic precipitator, wherein the particulate matter is collected in dry form for disposal.
The operating conditions of the spray dryer usually are adjusted so that substantially all the particulate matter, particularly the sulfite/sulfate compounds and the unreacted alkaline reagent, are dried before they reach the dryer walls or exhaust duct in order to prevent caking and/or plugging. Consequently, the residence time (the time for the reaction between sulfur dioxide and the alkaline reagent) in the spray dryer is relatively short, usually in the order of 3-7 seconds. As a result, the reaction efficiency typically is about 80-85% which means that the gas exhausting from the spray dryer still contains about 15-20% of the original amount of sulfur dioxide.
When a bag filter is used, the unreacted alkaline reagent collected on the bag surface can react with the sulfur dioxide remaining in the gas. However, only a small percentage of the remaining sulfur dioxide is removed because of the low moisture content of the unreacted alkaline reagent. Also, some fly ash usually remains entrained in the exhaust gases and must be removed by the bag filters. Fly ash tends to be quite harsh on fabric filters and can substantially reduce their effective life.
Such prior art dry scrubbing processes are exemplified in U.S. Pat. Nos. 3,932,587, 4,197,278, 4,279,873 and 4,324,770.
U.S. Pat. No. 3,852,410 discloses another dry scrubbing process wherein a finely divided soluble alkaline sodium compound is deposited on the filtering surfaces of a fabric type dust collector and a waste gas containing sulfur dioxide and particulate contaminants is passed through the dust collector. The particulate contaminants are filtered by the dust collector. A portion of the sulfur dioxide reacts with the alkaline sodium compound to form a cake which is periodically removed by shocking, vibrating, reverse air pulsing and the like.